(1) Field of the Invention
The present invention relates generally to semiconductor integrated circuit processing and more particularly to contact pad structures that resist intermetal dielectric cracking.
(2) Description of Prior Art
Bonding pads are the interfaces between the integrated circuits contained in semiconductor chips and the chip package. A large number of bonding pads is required to transmit power, ground and impute/output signals to the chip devices. It is thus important that the bonding pad yield be sufficiently high to ensure a high per chip yield.
The general bonding pad structure consists of metal layers, emanating from the terminals of the chip devices, separated by intermetal dielectric (IMD) layers that are often silicon oxide. Metal vias, W is often used, pass through the IMD layers connecting the metal layers. Wires are bonded to a bonding metal pattern and to the chip package forming electrical connections between the chip and the package. A passivation layer covers the surface, except over the bonding sites, to seal the chip from contaminants and for scratch protection.
One mode of failure of the bonding pad relates to the peeling of the wire from the metal pattern due to forces exerted especially during the bonding process. This has been addressed in U.S. Pat. No. 6,002,179 to Chan et al., who teach a bonding pad structure with increased peeling resistance and in U.S. Pat. No. 5,731,243 to Peng et al., who show a cleaning method to ensure contamination free bonding. Another failure mode that has been observed relates to bonding pad peel back, where forces during wire bonding may cause a delaminating of one or more of the underlying layers. Bonding pad structures that resist bond pad peeling have been disclosed in U.S. Pat. No. 6,025,277 to Chen et al. and in U.S. Pat. No. 5,707,894 to Hsiao.
Another failure mode involves cracking of the IMD. Referring to FIGS. 1a, 1b, and 1c, there is shown conventional via hole arrays. Regions 10 are IMD oxide layers, and regions 12 are metal filled via holes passing through the IMD. Cracks that are observed in the IMD are similar to cracks 22 depicted in FIG. 2. These are cracks that propagate along the IMD layer avoiding the metal of the vias. Once a small crack is initiated it will, under stresses prevalent in the layer during processing, grow extensively. Approaches to alleviate this cracking of the IMD focus on producing IMD layers with low residual stress. Composite silicon oxide layers serve this purpose and are used, such as HDP plus PETEOS layers. However, even with composite silicon oxide layers to reduce stress, the IMD layer is not strong enough to withstand stresses encountered during chip packaging and IMD cracking is still observed.